Mass spectrometry has become an essential analytical tool for the identification and quantification of both small molecules (e.g., drugs and their metabolites) and large molecules (e.g., polypeptides). Recently, there has been growing interest in the use of mass spectrometry for tissue imaging, which is the generation of spatially resolved maps depicting the distribution of one or more substances in a tissue sample. This technique has been described in numerous prior art references including, for example, U.S. Pat. Nos. 5,808,300 and 6,756,586, both to Caprioli. Mass spectral tissue imaging has a number of highly promising applications, including as a tool for the study of the metabolism and distribution of drugs in normal and cancerous tissue.
The basic process of mass spectral tissue imaging may be more easily explained with reference to FIG. 1, which depicts a tissue sample 102 held on a sample support plate 104. The tissue sample may be specially prepared, e.g., by application of an overlying matrix layer, to provide enhanced radiation absorption and consequent ion production. In accordance with the prior art technique, the operator specifies a rectangular area 106 defined by boundary 108 for mass spectral imaging. The boundary 108 will typically be sized and positioned such that the entire tissue sample lies within the area to be imaged. The mass spectral tissue image is generated by sequentially irradiating a large number of spatially separated target regions 112 (which may be ordered in a rectilinear grid with constant lateral spacing between adjacent target regions) that span the imaging area 106, and measuring the abundance of one or more molecules by analysis of the mass-to-charge ratios of the ions produced by irradiating each target region. A visual representation of the distribution of selected molecules may be constructed by assigning different colors or luminosities to ranges of molecular abundances; for example, a region having a high abundance of a selected molecule may be displayed as a bright area, whereas a region devoid of the selected molecule may be displayed as a dark area. It is notable that when the tissue sample has an irregular or otherwise non-rectangular shape, as depicted in FIG. 1, a substantial fraction of the target regions 112 will be located outside of the region occupied by the tissue sample, i.e., on the bare sample plate, and irradiation of such target regions will not yield meaningful data.
One of the major obstacles to the widespread use of tissue imaging as a standard industrial analytical technique is the lengthy analysis (scan) time required to obtain a mass spectral image. Generally, mass spectral imaging is performed at a uniform high spatial resolution over the entire tissue sample in order to ensure that areas of interest within the tissue sample (e.g., those areas where a highly differentiated analyte spatial distribution occurs) are adequately resolved. Generation of a mass spectral image for a typical tissue sample of 1 cm2 can require several hours or even days of instrument time. While these lengthy scan times may not be of paramount concern in research settings, there is a need to shorten the scan times before mass spectral imaging tools can be routinely and effectively deployed in pharmaceutical testing laboratories or other environments in which high sample throughput is required.
There have been a number of prior attempts to reduce mass spectral imaging scan times. These attempts have been largely focused on shortening the time required to acquire mass spectra at each target region (e.g., by reducing the number of laser pulses, increasing the laser repetition rate; or increasing the scan rate of the mass analyzer), or reducing the repositioning times associated with moving the laser beam from one target region to the next. However, such approaches may compromise the quality of the mass spectral data and/or require substantial modification of the hardware components to implement.